Mimetic drugs comprise an important part of the pharmacopoeia. They work by binding to receptors for hormones or other physiological mediators and either block (antagonize) or mimic (agonize) the effect of the natural ligand. Conventional methods for finding mimetic drugs are exceedingly expensive, involving the synthesis and testing of many individual compounds. The "mimitope library" contemplated under this proposal promises to greatly simplify the search for mimetic drugs, at least those that mimic protein or peptide ligands. A library of random peptides will be generated in a filamentous phage vector and screened for specific interaction with a cell surface DR1 antigen. A peptide sequence known to bind to this antigen will also be cloned into the vector as a positive control to demonstrate the feasibility of the screening methods. Random peptides that are selected by binding to the antigen will be characterized and produced synthetically and their affinities compared.